In general, there are several low-voltage power distribution systems such as a single-phase two-wire low-voltage power distribution system, a single-phase three-wire low-voltage power distribution system, and a three-phase four-wire low-voltage power distribution system (called a first conventional technique hereafter).
First, referring to the single-phase two-wire low-voltage power distribution system with reference to FIG. 1, FIG. 1 is a view illustrating a conventional single-phase two-wire low-voltage power distribution system. The system has an output voltage of 220V being drawn between a plus (+) terminal and a minus (−) terminal as illustrated in FIG. 1, wherein a grounding system with a grounding side connected to the minus terminal is applied.
Next, referring to the single-phase three-wire low-voltage power distribution system the majority of electrical equipment was configured for 110V in the past. However, as industrialization progressed rapidly, 220V was adopted. Accordingly, a new low-voltage power distribution system was needed in order to use existing 110V electrical equipment and 220V electrical equipment newly produced. To meet the need, the single-phase three-wire low-voltage power distribution system was introduced.
Next, referring to the three-phase four-wire low-voltage power distribution system, as industrialization progressed, the demand for three-phase electrical power appropriate for the large electric power load increased. Accordingly, the demand for the three-phase four-wire low-voltage power distribution system increased proportionally, since the distribution system can supply three-phase electrical power and single-phase at the same time. These days, as a result, the three-phase four-wire low-voltage power distribution system is widely used in a majority of low-voltage power distribution systems.
FIG. 2 is a view illustrating a conventional three-phase four-wire low-voltage power distribution system. Terminals of R, S, and T depicted in FIG. 2 are used by connecting them to a 380V three-phase electric motor, and 220V can be drawn by combining one of the terminals of R, S, and T with terminal N.
In the use of electricity in general, however, 220V electricity drawn from the single-phase two-wire low-voltage power distribution system, the single-phase three-wire low-voltage power distribution system, or the three-phase four-wire low-voltage power distribution system seems all the same, but characteristics of electricity drawn from each system are significantly different from one another from the point of view of the property of grounding system. Referring to the single-phase three-wire low-voltage power distribution system and the three-phase four-wire low-voltage power distribution system as examples, with reference to FIG. 3, is as follows.
FIG. 3 is a view illustrating electrical outlets in the conventional single-phase three-wire low-voltage power distribution system and the conventional three-phase four-wire low-voltage power distribution system.
First, in the case of the electrical outlet in the three-phase four-wire low-voltage power distribution system, 220V appears when the voltage between a terminal 1 and a terminal 3, earth, is measured and 0V appears when the voltage between a terminal 2 and the terminal 3 is measured.
Next, in the case of the electrical outlet in the single-phase three-wire low-voltage power distribution system, 110V appears when the voltage between a terminal 1 and the terminal 3, earth, is measured and 110V appears also when the voltage between a terminal 2 and the terminal 3 is measured.
As can be seen from the description above, a voltage to ground of the single-phase three-wire low-voltage power distribution system is different from a voltage to ground of the three-phase four-wire low-voltage power distribution system.
FIG. 4a is a view illustrating the result of a voltage to ground measured in the event of flooding in a conventional single-phase two-wire low-voltage power distribution system; FIG. 4b is a view illustrating the state of leakage current being measured by using an ammeter in the event of flooding in a conventional single-phase two-wire low-voltage power distribution system; FIG. 4c is an equivalent circuit of FIG. 4b; and FIG. 4d is a view illustrating the flow of momentary electric current in an equivalent circuit of FIG. 4c. 
As illustrated in FIG. 4a, two lines extend to a plastic pool from output terminals of plus and minus of 220V power supply lines drawn from the conventional single-phase two-wire low-voltage power distribution system; the two lines are immersed into water in the plastic pool; and one end side of another line is immersed into water in the pool away about 1 m from the two lines of plus and minus. When a voltage between water and an earth terminal is measured, 110 V, half of 220 V, appears. Consequently, when a person touches water and the earth terminal at the same time with his or her hand, the large amount of momentary electric current flows and may cause electric shock. As illustrated in FIG. 4b, at this time, electric current being leaked out can be measured with an ammeter installed between the water and the earth terminal.
Assuming about 3  of earth resistance exists between two places of the earth of FIG. 4b, an equivalent circuit of FIG. 4b can be depicted as FIG. 4c. 
At this time, referring to the flow of momentary electric current in an equivalent circuit, with reference to FIG. 4d, according to a direction of electromotive force by a voltage of 220 V, electric current flows along a path a and a path b, and then a path c. That is, in the flow of electric current, because they flow without offset of their forces each other, the ammeter displays a value indicating a huge amount of electric current. Therefore, when a person is positioned at the location of the ammeter (i.e., when a person touches water and the earth terminal at the same time) the large amount of momentary electric current flows and may cause electric shock.
The ground systems of the conventional single-phase two-wire low-voltage power distribution system and the three-phase four-wire low-voltage power distribution system have a critical drawback, namely: when an earth leakage circuit breaker operates normally in the event of flooding of electrical equipment, the electrical equipment loses the electrical power resulting in no operation; and when the earth leakage circuit breaker does not operate normally in the event of flooding of the electrical equipment, very high risk of an electric shock accident exists due to the large amount of electric current leaking to outside.
Meanwhile, Korean unexamined patent publication No. 10-2005-0037986 (called a second conventional technique, hereafter) depicts an anti-electric shock system in water immersion for preventing accidents due to earth leakage or electric shock by absorbing a leakage current leaking out from a bare current carrying part (current carrying part due to the absence of a sheath), when electrical equipment is flooded while electric current is carried. The anti-electric shock system in water immersion of the second conventional technique is illustrated into several embodiments. A common feature of the embodiments is that a flat plate metal board having area large enough to cover all other devices such as circuit breakers, stabilizer, etc. including a connection terminal board disposed with exposed connection terminals (single-phase connection terminal P, neutral point terminal N, earth terminal E) is connected to the neutral terminal (N) or the earth terminal (E), wherein the flat plate metal board is disposed at the bottom surface of the connection terminal board, circuit breakers, stabilizers, etc. while being electrically connected to the terminals of N or E.
According to a description of the second conventional technique, when exposed connection terminals of the connection terminal board are immersed into water, thanks to the configuration described, most electric current leaking out through bare current carrying part flows via the flat plate metal board. Therefore, when a person touches water, intensity of electric current flowing through human body is so weak that accidents due to electric shock or earth leakage can be prevented.
However, according to an experiment performed by manufacturing an anti-electric shock system in water immersion identical with the second conventional technique, the second conventional technique has a critical vulnerable point.
Firstly, in order to realize a prevention effect of earth leakage and electric shock, the metal board for the prevention of earth leakage should be connected to the neutral point terminal of AC power supply, but perfect assurance of this is problematic. According to the description of second conventional technique, in order to realize a prevention effect of earth leakage in the event of flooding, the flat plate metal board should be connected to the neutral point terminal or earth terminal of AC power supply. One way to achieve this is by, when the connection terminal board is installed, locating in advance the first power supply line that is connected to the neutral point terminal of power supply side out of two single-phase (1P) AC power supply lines, whereby the located line is connected to connection terminal to which flat plate metal board is connected, and the other second power supply line is connected to remaining connection terminal. However, this approach has several problems: location of the first power supply line that is connected to the neutral point terminal of power supply side is cumbersome; the prevention of earth leakage and electric shock cannot be achieved when connection is not performed properly with right line to the right terminal; and electrical power is wasted by connecting the electrical power load all the time to the power supply side even in the case the electrical power load does not need electrical power. To connect the electrical power load to the electrical power supply side only for the duration required as necessary, a plug and an electrical outlet may be considered to be disposed between the electrical power supply side and the connection terminal board. In this case, the path the metal board is electrically connected to the electrical power supply side becomes the terminal of the connection terminal board→ the plug terminal→ the electrical outlet. At this time, for the flat plate metal board to be connected to the neutral point terminal of the AC power supply, the plug terminal connected to the first connection terminal (J1) of the connection terminal board to which the flat plate metal board is connected should be perfectly assured to connect with electrical outlet terminal connected to the neutral point terminal of the AC power supply side. The plug has two plug terminals (IN1, IN2), which look the same, and one earth terminal (G), each electrically connected to each of three terminals of the connection terminal board. Furthermore, two electrical outlet terminals, to which AC power supply is provided, that is, the first electrical outlet terminal (N) connected to the neutral terminal of the AC power supply and the second terminal (R) connected to the single-phase voltage terminal look the same as well. Therefore, to correctly connect the first plug terminal (IN1) with the first electrical outlet terminal (N) when a user puts the plug into the electrical outlet, the user should know which one of the two plug terminals is the first plug terminal (N) and which one is the first electrical outlet terminal (N). In reality, however, assurance of these conditions is very difficult. Even a user who knows polarities of the plug and the electrical outlet terminal may commit a mistake of not connecting with the right polarity when not paying attention. Marking polarity on the plug terminal and the electrical outlet terminal is one way to prevent the user error, but a user who does not know this may put the plug into the electrical outlet. Considering the possibility of errors induced by inadvertence, this approach also has drawbacks.
Also, even though the second conventional technique describes the same effect is achieved when the metal board for the prevention of the earth leakage is connected to the earth terminal (E), according to the experiment, when the metal board for the prevention of the earth leakage is connected to the earth terminal (E), not to the neutral point terminal (N), the desired effect of preventing earth leakage and electric shock is unable to be achieved.
Secondly, a conductive metal board proposed by the second conventional technique is unable to provide prevention function of earth leakage and electric shock in the event of flooding, which is a fact different from what the second conventional technique claims. According to the results verified through various tests, the cause was identified as the flat plate structure of the conductive metal board for the prevention of earth leakage. According to the experiment, as proposed by the second conventional technique, the configuration, with the large flat plate metal board disposed beneath the connection terminal board and so on, causes an increase in the amount of leaking electric current in a few seconds to several ten seconds after the connection terminal board is immersed into water. This causes the earth leakage circuit breaker to actuate and then the electrical power supply to electrical power load is cut. At this time, a person felt the impact of electric shock as he put his hand into water that the connection terminal board was immersed in. The causes of this are: the distance between the conductive metal board for the prevention of earth leakage and the second connection terminal connected to the single-phase voltage terminal is excessively far; and the body of the connection terminal board consisted of insulating material disposed between them contributes to increase the resistance value between them by disturbing the electric current to flow via the shortest path. Consequently, even though a small portion of the electric current leaking out from the second connection terminal flows into the conductive metal board for the prevention of earth leakage, the remaining substantial amount of electric current leaks out to other place. Though the second conventional technique proposed the size of the flat plate conductive board for the prevention of earth leakage as 50 ×30  when operational voltage is 380 V, according to the experiment, when a conductive board far bigger than the above (for example, 60 ×60 ) is used, the earth leakage circuit breaker is actuated even though the breaker actuation time became a little longer. As such, it was recognized that the problem could not be resolved by increasing the size of the conductive board. In reality, due to the limitation of the space where the conductive board can be installed, the size of the conductive board cannot be increased indefinitely. Therefore, the second conventional technique has a drawback that cannot be resolved by increasing the size of the conductive board.
As described above, the first conventional technique has the following problems: when an earth leakage circuit breaker operates normally in the event of flooding of electrical equipment, the electrical equipment loses the electrical power resulting in no operation; and when the earth leakage circuit breaker does not operate normally in the event of flooding of the electrical equipment, very high risk of an electric shock accident exists due to the large amount of electric current leaking to outside. The second conventional technique has a problem of being unable to provide prevention effect of earth leakage and electric shock. To resolve these problems, a third conventional technique (refer to FIG. 5 to FIG. 7) described below has been developed by the inventor.
FIG. 5 is a view illustrating a transformer using a conventional neutral grounding system and a method therefor.
As illustrated in FIG. 5, the third conventional technique includes the transformer 50 using a conventional neutral grounding system in the single-phase two-wire low-voltage power distribution system. That is, the third conventional technique uses a neutral grounding system connecting a neutral point 51 (middle position of a secondary winding) of the secondary winding (winding of the output side) to a ground wire 52 in the single-phase two-wire low-voltage power distribution system comprised of a single-phase (1P) input side and a two-wire output side. Other transformation technologies are not described further in detail here since they are related arts.
As illustrated in FIG. 5, when the neutral grounding system is applied in the single-phase two-wire low-voltage power distribution system, in the event of flooding of exposed terminals of electrical equipment, electric current flows between plus polarity terminals, i.e., electric current flows from plus (+) polarity terminal to minus (−) polarity terminal whereby their respective forces offset each other, resulting in almost no leaking electric current to outside except the area between the plus polarity terminals and a vicinity thereof (refer to FIG. 6a to FIG. 6d, described later).
FIG. 6a is a view illustrating the result of a voltage to ground measured in the event of flooding in a conventional single-phase two-wire low-voltage power distribution system, wherein the system uses a neutral grounding system; FIG. 6b is a view illustrating the state of earth leakage being measured by using an ammeter in the event of flooding in a conventional single-phase two-wire low-voltage power distribution system, wherein the system uses a neutral grounding system; 6c is an equivalent circuit of FIG. 6b; and FIG. 6d is a view illustrating the flow of momentary current in an equivalent circuit of FIG. 6c. 
As illustrated in FIG. 6a, two lines extend to a plastic pool from output terminals of plus and minus of 220V power supply lines drawn from the neutral grounding system in the single-phase two-wire low-voltage power distribution system; the two lines are immersed into water in the plastic pool; and one end side of another line is immersed into water in the pool away about 10  from the two lines of plus and minus. When a voltage between water and the earth terminal is measured, a voltage equal to or less than 10V appears. More specifically, a voltage of 4V to 10V appears. Therefore, since it is generally known that electric shock does not occur at a voltage less than 30 V, when a person touches the water and the earth terminal at the same time with his or her hand, a negligible amount of electric current flows and the electric shock is prevented. As illustrated in FIG. 6b, it can be verified at this time that almost no electric current flows by measuring the leakage current with the ammeter installed between water and the earth terminal.
Assuming about 3  of earth resistance between two places of the earth of FIG. 6b, an equivalent circuit of FIG. 6b can be depicted as FIG. 6c. 
At this time, referring to the flow of momentary current in an equivalent circuit, with reference to FIG. 6d, according to a direction (for example, top to bottom in FIG. 6d) of electromotive force by a voltage of 220 V, electric current flows along a path g and then a path i through the water. Examining this in more detail shows the voltage between the terminal d and the terminal e is 220V. Since a potential difference between the terminal d and the terminal e is 220V and a first closed loop (path g−conducting wire of water−path i) is formed with water in the plastic pool as a conducting wire like this, naturally current by the voltage of 220V flows along the first closed loop.
Meanwhile, the voltage between the terminal d and the terminal f and between the terminal f and the terminal e is 110 V, respectively. Like this, a potential difference between the terminal d and the terminal f and between the terminal f and the terminal e is 110 V, respectively, and a second closed loop (path g−conducting wire of water−path h) and a third closed loop (path h−conducting wire of water−path i) as a conducting wire is formed with water in the plastic pool, respectively. It would seem natural that electric current flows along the second closed loop and the third closed loop. However, almost no electric current flows since the voltages between the terminal d and the terminal f, and between the terminal f and the terminal e are identical with 110 V, but have opposite directions of electromotive forces from each other. That is, regarding the neutral point 51 as a reference point, since the upper direction 110V and lower direction 110V are identical each other and directions of electromotive forces by voltages of both sides are opposite each other from the point of ammeter's view, their forces offset each other almost completely and almost no electric current flows, thus the ammeter display indicates nearly zero.
Consequentially, when single-phase 220V of a neutral grounding system in the single-phase two-wire low-voltage power distribution system is applied to electrical equipment that may be exposed to flooding, a risk of electric shock can be greatly reduced when earth leakage circuit breaker does not operate due to fault or even primary side of the earth leakage circuit breaker is immersed into water. That is, electric shock can be fundamentally prevented by applying the neutral grounding system in the single-phase two-wire low-voltage power distribution system, wherein almost no leakage current flows out from the terminals to outside in the event that exposed terminals of electrical equipment are immersed into water
FIG. 7 is a view to explain a means preventing an operation of a conventional earth leakage circuit breaker, wherein breaking operation prevention unit 720 is integrally embodied with earth leakage circuit breaker.
In general, the earth leakage circuit breaker 710 is a wiring appliance that automatically cuts the electricity by sensing a state in advance, wherein the state is when an input voltage higher than the nominal voltage is applied or earth leakage occurs at the electrical equipment. At this time, the earth leakage circuit breaker 710 should perform breaking operation within 0.03 second after the occurrence of earth leakage in the case of human body standard and should perform breaking operation within 0.1 second after the occurrence of earth leakage in the case of industrial standard. Since the earth leakage circuit breaker 710 like this is not to be described further in detail here since it is a related art. Only, the earth leakage circuit breaker 710 is connected to the power supply outputting from transformer 50 depicted in FIG. 5 and performs fundamentally the function to support the exposed terminal 721 of plus connected to the plus terminal and the exposed terminal 722 of minus connected to the minus terminal. In addition, it is desirable that the earth leakage circuit breaker 710 is waterproof.
At this time, earth leakage may occur in a general state other than flooding and in the event of flooding when the electrical equipment is immersed into water. When earth leakage occurs like this, the earth leakage circuit breaker 710 cuts the electrical power by actuating normally and the operation of electrical equipment that follows is stopped.
Meanwhile, as described above by referring to FIG. 5, and FIG. 6a to FIG. 6d, if it is possible to prevent the electric shock in the event that the exposed terminals of the electrical equipment are immersed into water by reducing greatly the leakage current leaking out from those terminals, it would be desirable to make such electrical equipment (especially, street light, traffic signal control unit, electrical outlet for the facility in the basement, agricultural equipment, etc.) operate normally by preventing the breaking operation of the earth leakage circuit breaker in the case of the occurrence of the electric shock due to the event of flooding.
Therefore, the third conventional technique includes the breaking operation prevention unit 720 to prevent the breaking operation of the earth leakage circuit breaker 710 by immersing the exposed plus and minus terminals into the water within the predetermined time (e.g., within 0.03 second), that is almost simultaneously.
At this time, the breaking operation prevention unit 720 prevents the breaking operation of the earth leakage circuit breaker 710: by installing the exposed terminal 721 of plus connected to the plus terminal and the exposed terminal 722 of minus connected to the minus terminal of the earth leakage circuit breaker 710 to space apart at a predetermined distance (for example, 7  to 8 ) between them and to keep balance of the bottoms thereof; and by allowing electric current to flow from the exposed terminal 721 of plus to the exposed terminal 722 of minus through the immersion of the exposed terminal 721 of plus and the exposed terminal 722 of minus into water within a predetermined time. That is, the breaking operation prevention unit 720 can prevent breaking operation by making the earth leakage circuit breaker 710 not to sense the electric shock state due to the event of flooding by making the bottoms of the exposed terminal 721 of plus and the exposed terminal 722 of minus be balanced (to be balanced the bottoms by using leveler when installed), thus they can be immersed into water simultaneously.
In addition, the breaking operation prevention unit 720 further includes a supporting structure 723 to maintain separation distance and balance of bottoms by supporting the exposed terminal 721 of plus and the exposed terminal 722 of minus. The breaking operation prevention unit 720 further includes a protection case 724 to protect the breaking operation prevention unit 720 from the velocity of the water, wherein the protection case 724 further includes a foreign substance inflow prevention unit 725. The protection case 724 further includes an air exhaust unit 726.
As such, the third conventional technique does not produce a leakage current even in the event that the exposed terminals are immersed into water when the neutral grounding system is applied in the single-phase two-wire low-voltage power distribution system.
Furthermore, in the conventional technique in most cases, usually, the amount of electric current between two power supply lines connected to electric power load is maintained in a balanced state, provided the amount of electric current between the lines is not balanced due to a leakage current, thus it has a problem that breaking operation is actuated and electric shock occurs.